| With the intensification of vegetation evapotranspiration triggered by rapid warming in recent decades,forest growth is gradually constrained by climatic aridification.Therefore,mastering the change pattern of tree growth and exploring the relationship between tree radial growth and meteorological factors can help to understand the response pattern of tree growth to climate change,so as to achieve more accurate prediction of forest dynamics in the context of future climate change.The purpose of this paper is to systematically investigate the change pattern of stem radial growth of Betula platyphylla in semi-arid regions of China and its relationship with environmental factors,and to provide a theoretical basis for further understanding the response of tree growth to climate change in semi-arid regions.In this study,we conducted two consecutive years(2021-2022)of tree stem radial growth change monitoring in natural forests of Betula platyphylla and simultaneous measurement of hydrothermal factors to reveal the growth pattern of tree stem radial growth change in semi-arid areas,and analyzed the effect of hydrothermal factors We also analyzed the influence of water and heat factors on the radial growth dynamics of tree stems.The results of the study are as follows:(1)The diurnal dynamics of stem radial changes in semi-arid Betula platyphylla are closely related to the meteorological conditions.In sunny days,the radial change of the stem was characterized by daytime contraction and nighttime expansion;in rainy days,the "day contraction and night expansion" pattern of the radial change of the stem was not obvious,but showed a "stepped" rising or continuous rising trend.At the same time,the daily radial variation of Betula platyphylla stem under 0-10 mm,10-25 mm,25-50 mm and >50mm rainfall levels was 1.60,2.51,1.78 and 1.31 times higher than that under no rainfall conditions(0mm),respectively.The daily dynamics of the radial variation of Betula platyphylla stem in the semiarid zone in different months was characterized by inverse phase variation during the growing season and non-growing season.The daily radial variation of Betula platyphylla stem showed daytime contraction and nighttime expansion during the growing season(May-September),while it showed daytime expansion and nighttime contraction during the non-growing season(October of the previous year-April of the following year).From different seasons,the daily radial variation of Betula platyphylla stems in the nongrowing season was greater than that in the growing season;from different years,the daily radial variation in most months was greater in 2021(wet year,current year’s rainfall > 1.4 times multi-year average rainfall)than in 2022(dry year,current year’s rainfall < multi-year average rainfall).(2)By comparing the spatial variability of the radial growth characteristics of Betula platyphylla in the three typical study areas,it was found that the radial growth of birch in the Saihanba study area was mainly regulated by temperature due to its higher altitude,lower average annual temperature and relatively humid climatic conditions.Compared with the other two study areas,the Betula platyphylla in the Saihanba study area had the earliest growth onset and the earliest growth end,and the lowest growth volume and growth rate.In contrast,both the drier Hanshan study site and the driest study site in Saihanwula showed long growth duration,high growth volume,and high growth rate in 2021(wet year).However,in 2022(drought year),the start and end of radial growth of Betula platyphylla stems in both the Hanshan study site and the Saihanwula study site were significantly earlier,and the Saihanwula study site was significantly affected by drought,with a greater advance in the end of growth.In the fall of 2022,due to a significant decrease in summer precipitation and decreasing soil moisture,the growth of Betula platyphylla was limited by moisture conditions,resulting in an earlier end of growth time and a decrease in total annual growth.(3)At the whole growing season scale,the radial growth of Betula platyphylla Suk.in the study area was always significantly influenced by temperature,while the sensitivity to moisture was relatively low.However,after dividing the growth stages according to the radial growth rate,moisture(especially soil moisture)had a significant effect on the radial growth of Betula platyphylla and varied significantly among growth stages.During the initiation phase(early May-mid June),the correlation between Betula platyphylla and precipitation was low in the three study areas,while the relationship with soil moisture showed significant differences among years: in 2021,the relationship between Betula platyphylla radial growth dynamics and soil moisture was significantly positive,but in 2022 the relationship was significantly negative,which may be due to the fact that the radial growth of Betula platyphylla at this time mainly consumed the water left in the soil during the previous season or the previous year.This may be due to the fact that the radial growth of Betula platyphylla at this time mainly consumed the water retained in the soil in the previous season or the previous year.In the rapid growth stage(mid-June to early August),soil temperature and soil moisture were the most important environmental factors affecting the radial growth of Betula platyphylla in the study area,among which,soil moisture was not replenished in time due to reduced precipitation in the summer of 2022.In the slow-growth stage(early August-late September),water and heat factors jointly affected the radial growth of Betula platyphylla,and the correlation size did not show a clear pattern between the two years,which indicates that in addition to environmental factors,the biological phenological rhythm also affects the radial growth in this stage.This suggests that,in addition to environmental factors,the phenological rhythm of the organism also affects the radial growth at this stage. |
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